1. Field of the Invention
The present invention generally relates to a photodetector. More particularly, the present invention relates to a far infrared photodetector utilizing a mechanism of detection based on free carrier absorption and internal photoemission over the bandgap offset of a heterojunction.
2. Description of the Related Art
Far infrared (hereinafter “FIR”) detectors are of interest for various astronomy applications such as the Stratospheric Observatory For Infrared Astronomy (SOFIA) program and Explorer missions. Stressed Ge[1] (hereinafter “[n]” referring to the nth reference in the attached list of references at the end of the specification) and blocked impurity band[2] detectors have been studied for almost 20 years as FIR detectors without being successful in making large arrays. Due to the material constraints in Ge that limit their use in arrays Si and GaAs homojunction interfacial workfunction internal photoemission infrared photodetectors (hereinafter “HIWIP”) have been studied as an alternative detector structure.[3, 4] HIWIP detectors include successive highly doped emitter layers and undoped barrier layers. Detection takes place by free carrier absorption in the emitter layers followed by the internal photoemission of photoexcited carriers across the barrier and collection.[5] The cutoff wavelength is determined by the workfunction at the interface which is due to the bandgap narrowing caused by the doping in the emitter. By adjusting the device parameters, mainly the doping concentration in the emitter region, the cutoff wavelength may be tailored to the desired range.
HIWIP detectors have shown high responsivity and good detectivity in this range, but have their limitations. One is the high dark current associated with higher doping required for longer wavelength operation. Also the highest quantum efficiency reported was around 12.5% for a 20 layer structure.[6]. The workfunction in HIWIPs is due to the bandgap narrowing effect in the highly doped emitter regions. High density theory, where only the dopant type (n or p) is considered but not the specific impurities, has been used to calculate the workfunction associated with doping concentration.[5] As the concentration is increased, the doping accuracy required to achieve a consistent workfunction for detection at wavelengths beyond 200 μm becomes more stringent. In addition, the most common p-dopant, beryllium diffuses spontaneously at the concentrations required for response beyond 200 microns. The diffusion problems may be eliminated by using carbon as the dopant. However, even with the reduced diffusion, the high precision in doping density, and the uncertainty associated with the bandgap narrowing still place potential limits on the use of HIWIPs. Another issue might be the direct transitions in the p-type (heavy hole and light hole) which is favored over indirect transitions. As the doping increased, the energy gap between the two hole bands increases, reducing the associated cutoff wavelength.
Thus, there is a need to develop a new photodetector. In particular, there is a need to develop a new type of far infrared photodetectors based on a new detection mechanism, giving high quantum efficiency.